1. Field of the Invention
The present invention relates generally to ring lasers and more particularly to ring lasers which may be used as gyroscopes to sense rate of rotation. Most particularly, the invention relates to ring lasers having a Faraday cell biasing system to prevent mode locking, or phase locking, at rotation rates which are within the range of interest.
2. Description of the Prior Art
In a ring laser gyro, an angular rate is measured by light waves traveling in a closed optical path commonly defined by three or four reflectors. Two laser beams, sustained by the optical gain provided by a gas discharge, propagate in clockwise and counterclockwise directions around the ring. With no rotation about the input axis, the frequencies of oscillation of the two counter-propagating beams are the same. Rotation of the gyro about its input axis (perpendicular to the plane of the enclosed optical path) in either direction causes an increase in cavity length for the beam traveling in the direction of rotation and a decrease in cavity length for the beam traveling against the direction of rotation.
Consequently, the frequency in each beam must shift slightly to maintain an integral number of wavelengths in each beam, a basic requirement to sustain laser oscillation. The frequencies of the two beams are then unequal by an amount proportional to the rotation rate of the gyro.
A beat signal is produced when the two counterrotating beams are properly combined. For rotation about an axis perpendicular to the plane of the cavity, the frequency of the beat signal will indicate the rotational rate applied to the cavity.
Ring lasers known in the art typically employ mechanical rotation or Faraday effect bias to separate the frequencies of counter-propagating laser beams sufficiently to prevent mode locking between the beams, particularly at low rotation rates. Periodic reversal of the bias, viz, modulation of the bias, is employed to minimize sensitivity to sources of bias drift and to provide partial cancellation of mode pulling and backscatter errors. The mechanical bias technique is undesirable from the standpoint of employing moving parts sensitive to the stresses of high acceleration. Modulation by periodic reversal of the magnetic field in a Faraday cell typically requires substantial amounts of electrical power and produces concomitant undesirable heating.
U.S. Pat. No. 3,617,129; Skolnick; Interferometric Optical Isolator describes a directional anisotropy as one which provides a different optical path length for waves travelling in opposite directions on an optical path. For instance, the directional anisotropy may comprise a Faraday rotator flanked by quarter-wave plates. As is known, the Faraday rotator comprises a suitable material with a proper axial magnetic flux therein. For instance, for light in the visible and near visible spectrums, quartz is suitable; for infra red radiation indium antiminide or gallium arsenide may be used.
U.S. Pat. No. 3,807,866; Zingery; Ring Laser Gyroscope Having a Constant Output Beat-Frequency states that the substance involved in producing the Faraday effect may be a material such as lithium silicate with a large percentage of terbium.
U.S. Pat. No. 3,826,575; Walter Jr.; High Performance Ring Laser Gyroscope with Magneto-Optical Bias states that the Faraday cell is a common device for achieving magneto-optical bias. It consists of two quarter-wave plates which enclose an optical medium with a relatively large Verdet constant. This optical medium is then surrounded by an electromagnet or a permanent magnet to produce the necessary magnetic field intensity. The optical medium rotates the polarization plane of polarized light passing through it. Quartz, for example, which does not normally have this rotational property, acquires it when placed in a strong magnetic field. For quartz, flint glass, or another similar substance, the Faraday rotation for a given wavelength of light is proportional to the magnetic field intensity. However, given a field of fixed intensity, every light transmissive material will produce an amount of Faraday rotation. This quality is generally indicated by the number called a Verdet constant, as mentioned above.
U.S. Pat. No. 3,890,047; Warner; Differential Laser Gyro Employing Reflection Polarization Anisotropy states that a Faraday cell may comprise any material with a suitable Verdet constant which is provided with a suitable magnetic field. The directional anisotropy of the Faraday cell is provided by the magnetic field in the material. The material may be fused quartz or a properly chosen glass.
Paramagnetic glasses have been used for Faraday elements. The paramagnetic glasses form a dilute matrix for certain ions, typically rare earth ions such as, for example, europium 3+ or gadolinium which give a relatively large Faraday rotation at a high magnetic field intensity. Faraday rotations of from 0.01 to 0.02 degrees are reasonably attainable using these materials.
However, these Faraday rotations are insufficient for many practical applications. Although the mode locking region can be readily shifted enough to measure angular rotation rates up to 30 degrees per second using prior art materials, many applications require the measurement of rotation rates of up to 100 degrees per second and beyond.
To produce a magnetic field intensity sufficiently large to bias a ring laser using a paramagnetic glass Faraday element beyond 100 degrees per second would require an inconveniently large magnet coil. In addition, such a coil would generate more heat than can be conveniently dissipated. Furthermore, if a magnetic shielding technique was being used to reduce or eliminate the effect of stray fields such as, for example, the earth's magnetic field, the required shielding would be inconveniently bulky. Such a ring laser would lose its compatibility with miniaturized solid-state components.
The most pertinent publications known to applicant are listed herewith.
Ito et al: LPE Films of Bismuth-Substituted Bubble Garnet, IEEE Transactions on Magnetics, Vol. MAG-9, No. 3, September 1973, pp. 460-463, which discusses the isothermal liquid-phase epitaxy of bismuth substituted garnet films on a gadolinium gallium garnet substrate.
U.S. Pat. No. 3,980,949; Feldtkeller; Magneto-Optical Measuring Transducer for Very High Currents/Voltages which discloses a magneto-optical measuring transducer comprising an yttrium iron garnet layer on a gadolinium gallium garnet plate and reflective layers on the garnet layers wherein the direction of polarization of a polarized beam of light is rotated in response to a magnetic field created by a current to be measured.
U.S. Pat. No. 3,927,946; McClure; Ring Laser Frequency Biasing Mechanism which describes a ring laser cavity forming component including a non-reciprocal phase shift inducing mirror comprising thin films of a high reflectivity multilayer dielectric and a magnetically saturable layer deposited on a substrate. The magnetically saturable layer is constructed of a ferromagnetic material such as iron, nickel, or cobalt. Non-reciprocal phase shift is based on the Kerr magneto-optic effect which is known to be lossier than the Faraday effect. McClure is hereby incorporated by reference into this specification in its entirety for its discussion of the general problem of mode locking in ring lasers and for its discussion of the secondary problems which must be accounted for or resolved in an operational ring laser instrument.
U.S. Pat. No. 3,851,973; Macek; Ring Laser Magnetic Bias Mirror Compensated For Non-Reciprocal Loss which discloses a multilayer dielectric mirror for use with the magneto-optic device of McClure, discussed above, wherein at least one layer of the multilayer dielectric mirror has a thickness adjusted to eliminate non-reciprocal loss (or differential reflection).
R. D. Henry et al, "Bubble Dynamics in Multilayer Garnet Films," J. Appl. Phys., August 1976, 47, 3702, discloses a multilayer garnet film structure in which three layers of yttrium iron garnet are successively deposited by liquid phase epitaxy.
R. D. Henry et al, "Ferrimagnetic Garnets as Laser Gyro Faraday Elements," "Proceedings of the Technical Program, ElectroOptics/Laser 77 Conference & Exposition," Anaheim, Calif. Oct. 25, 26, 27, 1977, Industrial and Scientific Conference Management, Inc., 222 West Adams St., Chicago, Ill. 60606, which discloses and describes various embodiments of the invention disclosed in this specification. This paper is hereby incorporated by reference into this specification in its entirety.
E. C. Whitcomb et al, "Fabrication of Thin Film Magnetic Garnet Structures for Intra-Cavity Laser Applications," a paper presented to the 23rd Conference on Magnetism and Magnetic Materials, Nov. 8-11, 1977, Minneapolis, Minn., and published in Journal of Applied Physics, 49 (3), p. 1803, March 1978, which discusses the variation of the index of refraction of yttrium iron garnet by varying the iron to gallium ratio therein. This paper is hereby incorporated by reference into this specification in its entirety.